The goal of this study is to provide an in situ assessment of sponge-associated microbial responses to ocean acidification, extending upon previous work at the Papau New Guinea (PNG) vents. A reciprocal transplantation of sponges between the PNG vent site and control site was performed to examine whether the microbial flexibility and stability patterns previously observed in Coelocarteria singaporensis and Stylissa cf. flabelliformis are maintained when adult sponges are moved between sites with differing pH.